You will drop into this area many (many!) times from the trapdoors in Cleowyn's Palace on Level 2. There's nothing for it but to trudge back up as exits from here are few. Get back by using the pit at D5 to the King's Domicile and the elevator there to come back up (see those sections).
Info is patchy. One Troll Cleric tells you there must be an exit, stairs or teleport. The 36 cells are empty except for spirits. An Orc Ranger tells you that some have escaped the dungeon. If you find the rainbow bridge, you are near the exit. Another NPC tells you of the Wizards' Challenge (you'll find this at the Great Corridor on this level). It was created by two wizards to challenge the brave. It's on this level but you can't reach it from the Prison. Know that race and guild must work together if the challenge is to be overcome. Another adds that a challenge awaits the brave if you dare continue after finding what the guard Deldwin desires (referring to the fact that the Challenge is further down the corridors where you find the King's Pass). He also says that Chaos should walk with Chaos and Harmony with Harmony to find the rewards. The only other thing you will learn is that the Palace was built in the volcano as the King wanted to learn the secrets of the wizard Arnakkian. Many of the stones that form the Palace were taken from the wizard's castle which is why the dungeon is cursed.
So, what is here? Not a lot. Some doors can be opened with a blue lockpick (see my comments earlier about choice of lockpicks), many more forced open. There are monsters of all sorts, some easy, some tough. I had a major battle at I16 but only won a lousy orange lockpick! The most useful place to find is a teleport at D11 which leads to the Soldier's Quarters C13 which leads to the main area via another teleport. A useful short cut.